1. Field of the Invention
The present invention relates to a single-ended class-D amplifier, and particularly to a single-ended class-D amplifier with dual feedback loop scheme.
2. Description of Related Art
Amplifiers can be categorized as class-A amplifiers, class-B amplifiers, AB-class amplifiers and class-D amplifiers. As the development of semiconductor technology, the class-D amplifiers with low power consumption have been applied in widespread applications, such as sound reinforcement system.
In comparison to AB-class amplifiers using linear signals, class-D amplifiers use pulse width modulation (PWM) technique to drive an inductive load device, wherein the PWM technique involves audio signals, PWM switch signals and harmonic signals. The PWM switch signals are applied to alternately turn on and off switching transistors of the class-D amplifier. Because the switches are either fully on or fully off, the power losses in the output devices are significantly reduced to ensure high power efficiency.
With reference to FIG. 5, a conventional single-ended class-D amplifier (70) with open loop scheme comprises a gain amplifier (71), a comparator (72), an oscillator (73), a logic circuit (74) and an output driver (75).                With further reference to FIG. 7, the gain amplifier (71) has an input terminal (Vi) for receiving an analog audio signal, amplifies the analog audio signal and transmits the amplified audio signal to the comparator (72). Upon the receipt of the amplified audio signal, the comparator (72) refers to an oscillating signal of the oscillator (73) to produce PWM signals (P, N). The logic circuit (74) based on the PWM signals (P, N) controls the output driver (75). The output driver (75) is adapted to connect to an inductive load device (60) via a low pass filter (80). The inductive load device (60), for example a speaker, receives the audio signal from the low pass filter (80) and restores sound accordingly.        
With further reference to FIG. 6, the oscillating signal produced by the oscillator (73) is a high frequency signal. For example, the frequency of the oscillating signal may be 350 kHz if the audio signal is in the range from 20 Hz to 20 kHz. Noises (N1) existing in the audio signal can be shaped to higher frequency beyond the signal band, such that the noises (N1) can be eliminated by the low pass characteristic (81) of the low pass filter (80) to retain desired audio data (S1).
It is difficult to reduce the size of the class-D amplifier (70) since the class-D amplifier (70) requires an additional low pass filter (80) to filter the noises been shaped to higher frequency. Furthermore, amplification elements of the gain amplifier (71) and the comparator (72) will cause a noise floor this one kind of random noise. Therefore, when the audio signal is input to the gain amplifier (71), the dynamic noise is also amplified and incorporated into the amplified audio signals, which deteriorates the total harmonic distortion plus noise (THD+N) and signal to noise and distortion ratio (SNDR) of the audio signals.
With reference to FIG. 8, another conventional single-ended class-D amplifier (70a) with close loop scheme comprises a gain amplifier (71), a comparator (72), an oscillator (73), a logic circuit (74), an output driver (75) and a first-order feedback circuit (711).
The gain amplifier (71) has an input terminal (Vi) for receiving an analog audio signal, amplifies the analog audio signal and transmits the amplified audio signal to the comparator (72). Upon the receipt of the amplified audio signal, the comparator (72) refers to an oscillating signal from the oscillator (73) to produce PWM signals. The logic circuit (74) based on the PWM signals to generate driving signals to turn on or off the output driver (75).
The output driver (75) comprises a high-side switch circuit (751) and a low-side switch circuit (752) both connected to the logic circuit (74) to receive the driving signals. The high-side switch circuit (751) and the low-side switch circuit (752) are connected in series at a node as an output terminal (Do) of the class-D amplifier (70a) for connecting to an inductive load device (60).
The first-order feedback circuit (711) is connected between the node of the output driver (75) and the input terminal (Vi) of the gain amplifier (71).
Since the class-D amplifier (70a) has incorporated with the first-order feedback circuit (711), many non-ideal components in the class-D amplifier (70a) can be eliminated, including amplifier frequency limitations, amplifier noise, reference voltage noise, gain-bandwidth product limitations and switch device non-linearity. However, because the class-D amplifier (70a) only uses a single gain amplifier (71) to process the audio signal, the noise shaping capability of the class-D amplifier (70a) is limited.
To overcome the shortcomings, the present invention provides a single-ended class-D amplifier with the dual feedback loop scheme to mitigate or obviate the aforementioned problems.